Timing conditioning circuits used in dual data rate (DOR) register integrated circuits typically require a precise clock having a frequency variation of less than about +/−2.5% over process, voltage and temperature (PVT) variations. Typically, such a precise clock needs to be generated by the integrated circuit, but without an external reference frequency. This poses important design challenges for an on-chip oscillator given the fact that integrated circuit components such as capacitors and resistors can have variations of around +/−20% over process variations and +/−2% over temperature variations.
Some examples of oscillator circuits that utilize resistors and capacitors to generate periodic signals are disclosed in U.S. Pat. Nos. 8,232,846 and 8,902,008 to DeVita et al., entitled “Resistor Capacitor (RC) Oscillator.” In these oscillator circuits, a current generator, a capacitor and a comparator are provided along with switch and clock generator logic. Additional examples of RC-based oscillator circuits are disclosed in U.S. Pat. No. 6,825,735 to Chung, entitled “Power Supply Voltage and Temperature-Independent RC Oscillator Using Controllable Schmitt Trigger,” which utilizes a transition voltage generator circuit, a Schmitt trigger circuit, an RC delay circuit and a quantizer. The transition voltage generator circuit is disclosed as generating a high transition voltage and a low transition voltage. The high and low transition voltages are proportional to a power supply voltage. The Schmitt trigger circuit generates an output voltage having a first level when an input voltage becomes greater than the high transition voltage and having a second level when the input voltage becomes less than the low transition voltage. The RC delay circuit includes a resistor and a capacitor and generates the input voltage in response to the output voltage. The quantizer quantizes the input voltage to output a square-wave oscillation signal. Notwithstanding these RC-based oscillator circuits, there continues to be a need for oscillator circuits having higher degrees of precision when PVT variations are present.